Abstract: A common reference system is presented for the load surface and the strength surface of a tall building subjected to vertical and lateral loads. As an example, the different shapes of the strength surface are given for a simple space truss. In the preliminary design, the strength surface enables the engineer to select a system, which meets the requirements of the predicted load surface, by selecting the appropriate system parameters. In the final design, the comparison of the load and strength surface provides the safety margins for all load combinations in any direction.